Power supplies for electrical equipment are available as DC power supplies that provide a user the capability of switching between different voltage or current settings. DC power supplies of this type are generally available in apparatuses or systems that may be used to test electrical equipment at different voltage or current settings. The different voltage or current settings are implemented by converting a main DC voltage at one high level to the lower user-selected DC voltage. The main DC voltage may be provided on a DC rail powered by an AC/DC converter and, in some implementations, available for more than one DC/DC converter to tap into for the main DC power used to down-convert to the user-selected DC level.
One problem that arises in operation of user-adjustable DC power supplies is excess energy that remains at the DC power output when the DC power level is switched to a lower level. The excess energy is typically stored on a filter capacitor typically provided at the DC supply output. The DC supply output may also be connected to a load that may have energy stored up from operating at a higher DC setting when the power switch occurs. A similar problem may also occur when any DC power supply experiences a sudden change in its load. The excess energy must be dissipated safely to avoid damage to the load or, possibly to the AC/DC converter or the DC/DC converter.
One solution is to connect dissipative elements across the output of the DC power supply to discharge the output filters. The dissipative elements are preferably designed to switch rapidly in order to achieve fast discharge times. Using this approach, the dissipative elements, or cells, all of the energy stored in the converter's output filter as well as the customer's load could be dissipated while lowering the output voltage to the value set by the user. Sub-systems that employed this solution were known as down-programmers. Down-programmers are typically designed in accordance with the ratings and specifications of the power supply model in which they are to operate. The need to conform to the characteristics of the power supply includes the need to seamlessly integrate with converter's control circuits in order for the output to sufficiently track the settings of the user.
Some DC/DC converters in DC power supplies are designed with synchronous rectification and are thus able to conduct current in both directions. This capability allowed power supplies to discharge output capacitors in a very short time and provided the converters with a small static current sinking capability. Energy from the output capacitors or the user's load may be directed back on to the power rail and used by all other systems using the power rail. The AC/DC converter supplying the DC power to the rail is not bi-directional. When net positive energy is directed to the power rail, the DC voltage could climb positive. In order to prevent this, a sub-system known in the art as an automatic down-programmer (“ADP”) may be added to dissipate the excess net energy.
The ADP is simpler than typical down-programmers since it does not require any custom design for each power supply model. In addition, since the ADP dissipates the balance of the energy sent back to the rail after conversion and usage by other subsystems, it is not typically required to dissipate as much energy. The ADP is able to sink high currents for short periods of time, which is suitable for discharging output capacitors. It also serves as a static load when one is needed. However, the load capacity of the ADP is limited thermally in typical implementations because of airflow capacity. The ADP's thermal capacity is limited by the need for space for large heat sinks. The ADP is also affected by the proximity to other large power-dissipating components. In some implementations, the ADP affects precision measurement or control circuitry in the instrument. These issues reduce the sinking capability of the ADP to a fraction of the power sourcing capability of the power supply instrument.
In view of the foregoing, there is an ongoing need for methods of apparatuses for dissipating a more substantial amount of the power sourcing capability of the power supply instrument. There is also a need for methods and apparatuses that are easily configurable for operation with different power supplies.